This invention relates to a method for preparing superconductors having improved high field critical current densities. More specifically, this invention relates to a method for preparing superconductors having an equiaxed fine grain beta-tungsten structure. Still more specifically, this invention relates to a method for preparing superconductors having an equiaxed fine grain beta-tungsten structure by sputter deposition.
As the requirements for larger electromagnets with higher magnetic fields increase, the importance of superconducting magnets increases. Of particular importance in making these magnets are the "hard" or type II superconductors, that is, superconductors which remain superconductive in the presence of intense magnetic fields. Of the type II superconductors, those compounds of the "beta-tungsten" (A-15) structure are the most promising. Examples of these superconductors (hereinafter referred to as A-15 compounds) are NB.sub.3 Sn, V.sub.3 Ga, Nb.sub.3 Al and Nb.sub.3 Al.sub.0.75 Ge.sub.0.25.
High-field, A-15 compound superconductors are normally used in long lengths, either ribbon or fibers, or as coatings, and they are extremely difficult to make because they are very brittle and have substantially no plastic deformation characteristics. Diffusion processes have been developed for making Nb.sub.3 Sn and V.sub.3 Ga whereby a ribbon of niobium or vanadium is drawn slowly through molten tin or gallium; however, these methods have inherent drawbacks even for Nb.sub.3 Sn and V.sub.3 Ga. Moreover, the methods are unworkable for making niobium-aluminum-germanium (Nb.sub.3 Al.sub.1.sub.-x Ge.sub.x) because chemical stoichiometry is impossible to attain and a multitude of intermetallic phases form during processing which render the product unusable. Furthermore, diffusion processed products contain expensive niobium, vanadium and gallium in the carrier ribbon and they also contain second phases which do not contribute to the superconductor's properties. Devices made from such material contain unneeded bulk which may also reduce performance levels.
Compounds of this type have also been prepared by sputter-deposition onto heated substrates and by vacuum vapor deposition onto very high-temperature substrates. While these methods produced thin films of compounds of the desired composition, the compounds either cracked and were unusable or produced critical current densities which were too low to be useful.